Thermo-



Jan. 21, 1964 s. F. MALAKER ETAL 3,113,285

CASCADE CRYOGENIC SYSTEM Filed June 16. 1961 3 Sheets-Sheet 1 INVENTOR. STEPHEN F. MALAKER JOHN G. DAUNT ATTORNEY Jan. 21, 1964 s. F. MALAKER ETAL 3,118,285

CASCADE CRYOGENIC SYSTEM Filed June 16. 1961 3 Sheets-Sheet 2 ull" INVENTORS, STEPHEN F. MALAKER JOHN G. DAUNT ATTORNEY 1964 s. F. MALAKER- ETAL 3,118,285

CASCADE CRYOGENIC SYSTEM Filed June 16, 1961 3 Sheets-Sheet 5- CRYOGENIC COOLING OPEN CLOSED J EQ ROOM LIQUEFIED THERMO- THERMO- THERMO- AND TEMP. GASES DYNAMIC DYNAMlC ELECTRIC THERMO- ELECTRIC 2730K E I Q E E 225K I E E -so:c -so F WWW IllllllllllIllllllllllllllllllllll OXYGEN NITROGEN E 2o| I 3 HYDROGEN E E I 0 5 E 4.2 K

HELIUM E INVENTORS. 7 STEPHEN F. MALAKER JOHN G.DAUNT A TTORNE Y United States Patent 3,118,285 CASCADE CRYQGENIC SYSTEM Stephen 'F. Malaker, ll iountainside, Ni, and John G.

Baunt, Columbus, Ohio, assignors to Mala-keriLaboratories, -Inc., High Bridge, NJL, acorporation of New Jersey Filed June 16, 1961, Ser. No. 117,555 4'Claims. (Cl. 62-3) This invention relates to a closed-cycle cascade combination cooling system for maintaining temperatures down to 70 K. and below. More specifically, it deals with a novel combination of a cryogenic engine operating on a modified Stirling cycle, and a solid state thermo-electric cooling system, connected in cascade, for providing very 'low temperatures in a closedsystem.

Present missile and other airborne equipment are re quiring a closed-cycle refrigerator for maintenance of temperature down to 70 K.,and lower. Such a unit must be fiyable, lightweight, small, eflicient, highly reliable, attitude independent, and capable of repeated intermittent operation with rapid cool-down times. To .date, no reliable unit meeting all of these requirements has been developed, even for temperatures in the neighborhood of 80 K. Currently, open-cycle systems ,or liquid feed systems are used, all of which have features unsatisfactory for service use.

The present invention provides a system which meets the aforesaid requirements in a fairly economical manner. It involves the cascade combination of ,a cryogenic engine operating on a-modified Stirling cycle, and a thermo-elec- 'tric cooling system.

In copending application Serial No. 102,489, filed by Stephen 'F. Malaker and John G. Daut on April 12, 1961, now Patent No. 3,074,244, there is described an efficient miniature cryogenic engine, operating on a modified Stirling cycle, and capable of being airborne with ease for the purpose of cooling equipment, and the like. The original Stirling cycle, which was an ideal cycle, was based upon piston motions which would allow the four steps of the cycle, i.e., the isothermal compression, the constantvolume gas transfer from compressor space to expander space, the isothermal expansion, and the constant-volume gas transfer from expander space to compressor space, to be made independently, consecutively and cyclically. However, the pistons of the aforesaid cryogenic engine employed in the present invention, driven by a crank-shaft, move approximately harmonically, and, consequently, the cycle is not ideal. For this reason, the cycle is called a modified Stirling cycle. Such a system is capable of providing temperatures in the neighborhood of about 9.0 to 100 K., or even somewhat lower. It has been found that the cooling end of this system may be connected in cascade with a solid state thermo-electric cooling unit in a novel and efficient manner.

Heretofore, it has been considered that solid state thermo-electric devices would be too inefiicient for low temperature refrigeration, so that their usage has been limited to cooling in the range of about 250 K. Recently, however, it has been found that solid state thermo-electric systems, particularly the semiconductor types, exhibit a surprisingly high efliciency at low temperatures. For example, recent work on bismuth-antimony-lead alloy junctions indicate a figure of merit as high as 5X1.0 K. and further, they show a remarkably low figure of merit loss and, in fact, even a gain with temperature reduction, in comparison with the other types of thermoelectric systems. The most unique feature of the present invention is that, when combined in a tandem arrangement with the aforesaid cryogenic engine, a refrigerating system may be obtained which is surprisingly small in size CTl 3,118,285 Patented Jan. 21, 1964 and weight for the cooling output and range achieved, and the overall cooling efficiency is surprisingly high. Particularly advantageous are the combinations employing solid state thermo-electric systems employing groups 1V to VI compounds, e.g., those of bismuth and antimony, and especially with smalladditions of lead.

The invention will be more readily understood by reference .to the accompanying drawings in which a preferred embodiment is described andin which FIGURE 1 depicts a cross-sectional side view of a preferred embodiment of the present invention adapted to serve as an infra-red signal detector in a satellite, for example. FIGURE 2 presents a top view of the upper portion thereof. An enlarged perspective front view of a typical solid state thermo-electric junction, such as those employed in the :present invention, is illustrated in FIGURE 3, while FLIGUR-ES 4 and 5 are front views of pyramided assemblies of a number of such junctions in parallel .and seriesarrangement, respectively. FIGURE 6 illustrates .a cross-sectional front view of a pyramided parallel assembly of junctions connected with the cold output end of a cryogenic engine. FIGURE 7 presents a chart indicating the state of the art in the cryogenic field dealt with herein. Similar numerals refer to similar parts in the various figures.

Reference to FIGURE .7 will show the state of the cryogenic art with respect to low temperature realization in closed systems of interest here. In thefirst column there are indicated the cooling temperatures reached by liquefied gases shown in the second column. The third column illustrates the fact that open thermodynamic cycles can reach temperatures in the neighborhood of 77 K. On the other hand, closed thermodynamic cycles (as exemplied by cryogenic engines employing the modified Stirling cycle) can cool down to the neighborhood of K., as is illustrated by the fourth column. But thermo-electric systems, displayed schematically in the fifth column, can effectively cool only to about 225 K. Yet, as depicted in column 6, combinations of thermodynamic and thermoelectric systems may be made to cool 'far 'below 77 K., and even to the temperatures attained by liquid helium.

Thermoelectric cooling devices are composed of elements having different thermo-electric powers. Generally, they consist of one p-type and one n-type of semiconductor connected together to form a single or multiple junction. Passing an electric current through the junction produces :a -IPeltier effect .at .the junction which, with proper current direction, produces a cooling effect at that point. The high temperature end of the couple releases heat to its surroundings.

The efficiency of the cooling process is best evaluated by the coefficient of performance 7;, given by the relationship where Q is the heat absorbed by the device at the low temperature, and W is the-energy put into the device electrical'ly.

It can be shown that the maximum valueof 1;, using the first approximation theory, may be expressed as where T is the mean temperature, AT the temperature difference between the hot and cold ends, and Z is the overall averaged Figure of Merit for the couple.

The figure of merit, Z, is a function of the Seebacl: COEffiClfil'lt the volts per degree difference produced by the couple), and of the electrical and thermal conductivities 3 of the elements. If these parameters were temperature independent, Z could be expressed as follows:

a pn

which is the ultimate thermodynamic Carnot efiiciency of any reversible engine.

In practice, Z is far from infinite and is a complicated function of the properties of the materials, since, in general, apn, 6 and K are all temperature dependent. At any particular temperature, it is customary to evaluate a figure of merit for each element, designated as z, which is defined by the expression: 1

oz28 V Then, Z is obtained by appropriate element and tempera,- ture averaging Z and Z In practice, the value of Z for various functions has been calculated to be 0.1x 10 K. for chromel-constantan couples, 0.-l8 10 K. -for ordinary bismuth-antimony p-n junctions, and 2.0 =10 Kf for lead telluride p-n junctions. However, the really noteworthy feature is that, .whereas Z for the other junction falls in value as temperature is lowered, the value of Z for semiconductor junctions under consideration, after dropping somewhat at about 275 K., remains quite constant at lower temperatures. Since even bis-muth-antimony-lead alloy junctions can be made to yield a figure of merit (Z) as high as x10" K. at temperatures as low as 80 K., it is found that by connecting such a couple system with the cold end of a cryogenic engine operating on a modified Stirling cycle, unusually low temperatures in the neighborhood of 70 K. can be readily reached.

A typical solid state thermo-electric junction is illustrated in enlarged for-m in FIGURE 3 where numeral refers to the heat-absorbing electrode (cold junction). The n-type of semiconductor is designated by 11, the arrow indicating the direction of electron flow. The ptype of semi-conductor is designated as 12, and the arrow indicates the hole flow therein. The heat-dissipating electrode (hot junction) is shown as 13-43, and the arrows indicate the current flow through the junction.

These thermo-electric junctions can be connected with each other in parallel or in series, to pyramid their cooling effect. FIGURE 4 illustrates a parallel arrangement of thermo-electric junctions in which the cold junction 14 is the result of current passing through the pyramided three rows of junctions, electrically connected in contact arrangement, with the current entering electrode 15 of the hot junction and leaving at electrode '15 thereof, in the direction of the arrows.

A series arrangement of thermo-electric junctions is illustrated in FIGURE 5. There, the three rows of junctions are separated by heat-conducting electricalvinsulating sheets 16 and 16. The electrical current enters electrode 17 of the hot junction (as shown by the arrow), passes through connection 18 to the second row of junctions, thence through connect-ion 19 to electrode 21 of the cold junction 20, and out through electrode 22 of the cold junction, as indicated by the arrow. The choice of whether series or parallel arrangement is to be used is dependent upon voltage and current considerations.

Among materials suitable for the low temperature solid state thermo-electric junction materials, are included the metals and semiconductor compounds of groups IV to VI of the periodic table. For example, the p-type materials include Be Te Sb Te alloys, such as an alloy containing 75% Be Te and 25% Sb Te also AgSb Te alloys with group IV-VI compounds, and AgSbTealloyed with GeTe (e.g., 10% GeTe), and lead tell-uride junctions. For the n-type materials, there are included Bi Te Bi Se alloys, and solid solution alloys of binary compound semiconductors, all of which materials are now available on Serial No. 102,489, now Patent No. 3,074,244, although other similar units operating on a modified Stirling cycle would also be suitable for the purpose. In this engine, shaft 51 is driven by an electric motor (not shown), which is powered by a suitable source, such as a solar battery (not shown). This shaft is connected to cranks 52 and 52' on which are mounted piston rods 53 and 54, respectively, having a phase angle of about 90. These are disposed within housing '55. Projecting at right angles from the side Wall of housing '55 is compressor cylinder 56, in which rides compressor piston 57 attached to piston rod 54. The working gas compressed by piston 57 passes through perforated head 58 and regenerator 59 disposed on the end of cylinder 56. Regenerator 59 is filled with a metallic network such as a metal wool of high heat capacity serving to absorb compression heat from the gas leaving compression cylinder 56.

Also projecting at right angles from the side wall of housing 55, and disposed immediately adjacent to and parallel to cylinder 13, is expander cylinder 60 in which reciprocates expander piston 61 attached to the end of connecting rod 53. Gas leaving generator 59 passes through tube 62 which leads to the end of expander cylinder 6%. Both cylinders 56 and 60' are provided with cooling fins 63.

Housing 55 is provided with gas filling inlet 64- for introducing the working gas (hydrogen or helium) into the system. Needle valve 65 serves as a shut-off for inlet 64.

The solid state thermo-electric cooling and sensing system, designated generally as 30, is mounted on the connecting tube 62 of the cryogenic engine, since this is the cold or output end of the cryogenic engine 50. As is shown in FIGURE 5, there is first placed over the tube 62 a thin heat-conductive, electrical insulating sheet 26, such as one of anodized aluminum. On this sheet is mounted the pyramided thermo-electric junctions designated generally (FIG. 4) as 70. Although a parallel arrangement is depicted on the engine, a series arrangement, as shown in FIGURE 5, may be used, depending on current and voltage source availabilities. The junctions are joined together by soldering, welding or brazing, to form an integral unit which is fixed to the engine tube 62 by casting over said tube an insulating resin 78 preferably a polyfiuororesin referred to herein. Prior to casting the resin, the proper connections are made, such as connecting electrode 31 to terminal 28 by connecting link 71, and connecting electrode 31' with terminal 29 by wire 72. A glass cover 73 (FIG. I) may be employed to protect the thermo-electric pyramid 70. As shown in FIGURE 2, the cold and hot junctions of the thermoelectric system may be connected to terminals 74 and 75-, respectively, by Wires 76 and 77, respectively, to measure the infra-red effect upon the system for sensing purposes.

FIGURE 6 shows an enlarged cross-sectional side view of the thermo-electric junction connection to the cryogenic engine, employing a parallel arrangement of junctions. The cold end 62 of the cryogenic engine is separated from the thermo-electric system by a thin sheet 26 of heatconducting, electric-insulating material, such as anodized aluminum. On this sheet 26 is mounted the pyramided thermo-electric system 30 having cold junction 27. All of the thermo-electric junctions are welded, soldered, or brazed into a single rigid structure, the lower portion (at least) of which is imbedded in an insulating plastic 78, such as a fiuoropolymer, e.g., polytetra'lluoroethylene, polytrilluoromonochloroethylene, or the like. Electric terminal 29 serves as the negative lead, both of which may be connected to electrical source, such as a solar battery (not shown).

One highly valuable feature of the present invention is the fact that the thermoelectric unit may be employed to speed up the cooling of the cold end of the cryogenic engine to an equilibrium value. This is done by reversing the electrical leads of the thermo-electric junctions when the cryogenic engine is started. For example, lead 29 in FIGURE 6 may be made the inlet or positive terminal and lead 28 the negative terminal, which action causes the heretofore cold junction 27 now to serve as the hot junction, so that cooling takes place at electrodes 31, 31', etc., adjacent to the head or cold end of the cryogenic engine. When the engine has reached an equilibrium value, terminals 28 and 29 may be reversed by means of a switch, whereupon the cascade or tandem cooling action of the two systems begins to take efiect.

When in use, the cryogenic engine is started by connecting the motor (not shown) to the electric current source, such as a solar battery (not shown). This causes compression piston 57 to move upwardly to compress the gas in cylinder 56, while expander piston 61 is stationary at the outermost part of its stroke. Then, as compression piston 57 continues to move upwardly, expander piston 61 begins to move downwardly so that compressed gas from cylinder 56 passes through the multiple gas passages in perforated head 58, whereby maximum heat transfer is obtained with minimum pressure drop and dead volume. The heated gas, in passing through regenerator 5?, gives up its heat to the metal wool and then passes through tube 62 past the hot end electrodes 3131 of the thermo-electric junction system, and into cylinder 66 wherein expander piston 61 already is on its downward stroke. Thus, the gas is transferred at approximately constant volume between the two cylinders.

As compression piston 57 remains stationary at the end of its stroke, expander piston 61 begins to move downwardly, whereby heat is taken in the expander and tube 62 is cooled. Thereafter, as compression piston 57 moves downwardly, expander piston 61 moves upwardly and gas from cylinder 69 passes through tube 62 and regenerator 59 where it picks up heat as it enters cylinder 56. Expander piston 61 already is on its upward stroke and the gas is transferred between cylinders at approximately constant volume.

As this cycle is repeated, tube 62 becomes cooled. By reversing the flow of current through the thermo-electric system so that the flow will be from terminal 29 to terminal 28, the cold junction 27 becomes the hot junction and cooling takes place at the tube 62, which has the effect of speeding up the cooling action of engine 50. As

soon as equilibrium is reached, the current through terminals 28 and 29 is reversed, so that junction 27 becomes the cold junction, and electrodes 3131 form the hot junction, whereupon a tandem or cascade cooling action is effected causing the temperature of cold junction 27 to become much colder than tube 62, which now is cooled by the cryogenic engine.

We claim:

1. A cryogenic system \for producing very low temperatures comprising a cryogenic engine designed to operate on a modified Stirling cycle and having a cold output end, a sheet of heat-conducting, electrical insulating material disposed against said cold end, a pyramided series of solid state thermo-electric junctions having a hot junction and a cold junction, said hot junction being mounted on said sheet, whereby the refrigeration of the entire system is cascaded to said cold junction, and a plastic insulating casting encasing said cold end, said sheet, and at least the hot junction of said thermo-electric series in a manner to serve as an encapsulating mounting for the cold end of the system.

2. A cryogenic system for producing very low temperatures comprising a cryogenic engine designed to operate on a modified Stirling cycle and having a cold output end, a pyramided series of solid state thermo-electric junctions made of compounds of groups IV to VI of the periodic table and having a hot junction and a cold junction, said hot junction being mounted on said cold output end in heat-conducting, electrical-insulating relation therewith, whereby the refrigeration of the entire system is cascaded to said cold junction, and an electrical insulating casting encapsulating said cold output end and at least the hot junction of said thermo-electric series in a manner to serve as an encapsulating mounting for the cold end of the systern.

3. A cryogenic system for producing very low temperatures comprising a cryogenic engine designed to operate on a modified Stirling cycle and having a cold output end, a pyramided series of solid state thermo-electric junctions made of compounds of groups IV to VI of the periodic table together with small amounts of lead and having a hot junction and a cold junction, said hot junction being mounted on said cold output end in heat-conducting, electrical-insulation relation therewith, whereby the refrigeration of the entire system is cascaded to said cold junction, and an electrical insulating casting encapsulating said cold output end and at least the hot junction of said thermoelectric series in a manner to serve as an encapsulating mounting for the cold end of the system.

4. A method for producing very low temperatures comprising operating a cryogenic engine on a modified Stirling cycle to cool the output end of said engine, mounting on said end a hot junction of a pyramided senies of sol-id state thermoelectric junctions in heat-conducting, electrical-insulating relation therewith, passing an electrical current through said thermoelectric junctions in a direction such that the lower junction of said series in contact with said engine end is the cold junction of said series, continuing passing the electrical current until the cryogenic engine operation has reached a thenmal equilibrium, and thereafter reversing the direction of current through said series.

References Cited in the file of this patent UNITED STATES PATENTS 2,844,638 Lindenblad July 22, 1958 2,856,756 Kohler Oct. 21, 1958 2,964,912 Roeder Dec. 20, 1960 3,037,358 Scofield June 5, 1962 

1. A CRYOGENIC SYSTEM FOR PRODUCING VERY LOW TEMPERATURES COMPRISING A CRYOGENIC ENGINE DESIGNED TO OPERATE ON A MODIFIED STIRLING CYCLE AND HAVING A COLD OUTPUT END, A SHEET OF HEAT-CONDUCTING, ELECTRICAL INSULATING MATERIAL DISPOSED AGAINST SAID COLD END, A PYRAMIDED SERIES OF SOLID STATE THERMO-ELECTRIC JUNCTIONS HAVING A HOT JUNCTION AND A COLD JUNCTION, SAID HOT JUNCTION BEING MOUNTED ON SAID SHEET, WHEREBY THE REFRIGERATION OF THE ENTIRE SYSTEM IS CASCADED TO SAID COLD JUNCTION, AND A PLASTIC INSULATING CASTING ENCASING SAID COLD END, SAID SHEET, AND AT LEAST THE HOT JUNCTION OF SAID THERMO-ELECTRIC SERIES IN A MANNER TO SERVE AS AN ENCAPSULATING MOUNTING FOR THE COLD END OF THE SYSTEM. 